Since the traditional UT systems are well understood, and their basic technology easily accessible to both new manufacturers and their clients alike, it is very important to peel back the IP curtains on EMAT and what part of EMAT tech is directly translatable to current UT standards.
The simple fact that EMAT produces ultrasonic waves in steel is not enough to classify it as a conventional UT system. How those waves are produced, and possibly even more important how the reflections from defects are read by the system, is of outpost importance.
If EMAT is to be considered as a replacement of UT systems everywhere, then it should answer some basic UT type questions regarding its technology. More importantly, these questions should be asked of any EMAT manufacturer with clear answers provided.
Yes, API 5CT does not reference any specific ultrasonic testing technique to be used for inspection, but there are a few standards that are referenced in 5CT such as ASTM E213 and E273. As such a minimum signal to noise ration SNR of 3, and a repeatability of better than 10% should be maintained by the NDT system, to say the least. Testing from both sides with a minimum of 4 test heads, 2 ID and 2 OD, is another requirement in the referenced standards to be met as well. We elaborate on these requirements further down in the article.
For a traditional UT system (excluding PA to a degree), the size of the transducer crystal is on average around 10mm in diameter or there about. This means that a 1” long notch presented for calibration might as well be the length of a bus. This is the case in part due to a much smaller probe element compared to the notch size, producing a system response that is linear to the notch depth not its length and
EMAT sensors are of comparable size to the notch itself. This means the signal received from a notch is proportional to its length not depth, up to a notch length equal to the EMAT sensor length. Therefore, a shorter defect of identical depth will produce a lower signal.
It’s worth noting that inclusions, hook cracks, penetrators and the like are much smaller in size compared to the notch length. Just being able to detect a beautifully machined long notch doesn’t suffice as defects come in all shapes, sizes, lengths, and orientations.
Finding out the linearity of the system with the notch depth and NOT its length is critical when choosing an EMAT over a UT system.
Detecting notches and detecting typical defects to the manufacturing process is what sets the product quality apart from your competition. Ask your EMAT or UT equipment supplier to show you some typical defects found and perhaps some more exotic defects they picked up that aren’t the length or as well defined as the calibration notch. After all we all need to comply with API 5CT 10.15.4.a and b no?
When it comes to picking up notches, the specification requires testing of ERW welds, and other products, from both sides of the weld for the obvious reason that the typical defect is not so well positioned or orientated as a notch is. How many EMAT probes are required for your application to achieve ID, OD, longitudinal and transverse inspections from both sides of the weld? In an Off-Line system this dictates the size of the mechanics, untested ends, and a variety of other limiting factors to inspection or production efficiency.
Most EMAT manufacturers claim the system is not sensitive to surface quality, a claim that should be questioned when heavy signal filtering is done by proprietary algorithms to achieve the required SNR of 3 or better. Perhaps allowing the customer to see the unfiltered signal, as available with traditional UT systems, is a good feature to have on an EMAT unit.
The very nature of inducing Eddy currents into the material is subject to the laws of physics regardless of the EMAT system manufacturer. This makes it sensitive to the surface quality (just ask Eddy current system manufacturers). This process subsequently creates an ultrasonic signal in the material which is significantly attenuated when returning to the probe as compared to its original generated signal. The return signal will also be affected by the surface quality, further adding to the issues masked by proprietary filtering algorithms. Surface quality issues are plainly apparent in traditional UT systems and easily recognized on the screen during calibration and especially when needed during production.
Ultrasonic Beam Angle
In traditional UT systems the same beam (45deg) is used for both OD and ID notch calibration. This means the OD notch is usually detected by a set of probes that bounce the ultrasonic beam from the ID of the pipe up to the OD notch at 45deg. The advantages of this method are too many to enumerate, with mid-wall defect detection being of key importance when considering volumetric coverage of the entire weld.
Usually using an EMAT system, a separate angle (90deg) is used for the OD notch detection. If this method is used, it is important to ask the EMAT manufacturer why a different angle is used/needed for OD, compared to ID. After all a direct replacement to a traditional UT system should be able to behave the same or better than the system it proposes to replace.
More importantly, proof of complete volumetric weld coverage must be demanded of the EMAT or UT equipment manufacturer, especially when concerning heavy walls like 0.500”. This proof should be verified with real world examples, especially at those thicker walls and large pipe diameters.
Signal to Noise Ratios SNR
Much like turning up the volume on a radio, the static gets amplified at the same time as the signal you wish to hear. The specifications call for a signal to noise ration SNR of better than 3:1.
For traditional UT systems this can be easily verified as not much filtering is done to the signal if at all.
For EMAT however, the amount of filtering varies and how it is done is rarely available for scrutiny as it is locked behind patented IP. The variations in SNR both due to algorithm filtering and real-world use need to be documented and their source well understood. Filtering signals too heavily can lead to missed indication of potential defects developing or missing defects entirely.
Can an EMAT system detect a 1/16” through drilled hole TDH at the same time as the ID and OD calibrated 5% notches? And at what signal to noise ratio would this be possible? This would show the operator that the ultrasonic beam is pointing at the ID or OD when calibrated on a TDH.
This also poses a very good question of repeatability both during calibration, not usually an issue, and during inspection at mill speeds. Repeatability should be better than 10% according to ASTM E273.
Frequencies
As previously mentioned, the UT world is very well understood. The most common frequencies used in transducers are usually anywhere from 2.5MHz to 5MHz.
With lower frequency comes reduced sensitivity to short lived defects. Usually for this reason in the ERW world the most common frequencies used for traditional UT probes are 4MHz and 5 MHz. This allows the detection of very small and short-lived defects at high production speeds.
The most common EMAT probes used produce a frequency of 2.5MHz for a beam of angles up to 45deg. In the case of the above mentioned 90deg angle usually used for OD notch/defect detection, a much lower frequency of 1.3MHz (or there abouts) must be used. This is to produce surface waves, which are only active for a given wavelength thickness. This means that to have adequate coverage for the top half of the wall thickness, a lower frequency must be used, which is accompanied by decreased sensitivity, something we all learned in basic Level 1 and 2 UT training.
Coupling Checks
There are still many UT system manufacturers that, to this date, don’t understand the need for a correct coupling check. The need arises from the simple fact that there are many ways to lose a signal, not create it in the first place or not be able to receive a signal from a defect.
For this reason, the best coupling check is performed when 2 traditional ultrasonic probes are placed across the weld from each other. One transmits the ultrasonic signal and the other confirms its reception, validating not just the full volumetric testing of the weld, but also the fact that the produced ultrasonic signal penetrated the weld at the correct strength and angle.
Since most EMAT systems use only 2 probes, and these don’t communicate one with the other, it is very important to know exactly how it is known that the correct ultrasonic beam characteristics were obtained in the material tested. Has the frequency changed, maybe its strength or maybe the probe is slightly damaged in a way where either the signal strength is not adequate, or its angle changed. Even worse the signal might not cover the entire volumetric area of the weld allowing mid-wall defects to pass undetected.
Because with EMAT the ultrasonic signal isn’t produced or delivered in its “original” form but rather by Electro Magnetic Accosting induction, there are more chances and places for an ultrasonic beam not to be made correctly or received entirely. The more cogs in a system, the more chances for disaster, ergo more checks need to be performed. How is the ultrasonic beam verified on an EMAT system while in use not just on paper?
As a side note, for those of you thinking of resolving the issue of a wet pipe by using an EMAT system, akin to scratching your left ear with your right hand behind your head, consider how EMAT probes are cooled from heat absorption during testing and during the signal production. Most would be hard pressed to know that EMAT probes usually come with a rather sensitive and expensive probe cooling system akin to that used in the welder or annealers or PA systems. Keep in mind that traditional UT systems (PA excluded) use mill coolant already wetting the pipe and easily available, instead of the mineral free water system needed by an EMAT probe.
A traditional UT system usually has what’s known as a Pulse Repetition Frequency or PRF for short. On average 2000 PRF per channel is a common capability, if not please reconsider the choice of UT system supplier.
The importance of PRF cannot be understated, especially when it comes to On-Line ultrasonic testing at mill speeds such as 200ft/min and higher. The reason for this is detectability of short-lived defects. Transducer crystal diameter, PRF and mill speeds dictate how many times an ultrasonic pulse “hits” a defect before that defect moves a given distance down the mill.
Case in point, if the PRF is too low, a defect can be “seen” by the same probe at its beginning and at its end without triggering a defect alarm. However, if the PRF is high enough the same defect mentioned will also be “seen” in its entirety (often multiple times) returning a much large signature to the system and triggering an alarm before moving a given distance down the mill.
This PRF is often omitted in specifications on an EMAT system, but it should be present in the required manufacturer’s calculations of the weld coverage side to side of center and volumetric coverage as well. After all they are pulsing their signals just like a traditional UT is.
When it comes to the number of EMAT probes used for a specific application, the manufacturer should specify if the same probe is used to generate the 45deg waves as well as the 90deg waves. If that is the case, how many times per second is this mode changing? This should also be a part of the manufacturers required coverage calculations, amongst other things. These calculations should indicate the EMAT or UT system max testing speed, valuable information for mills that do produce pipe a 200ft/min+.
For Off-Line inspection of ERW pipe, PRF isn’t such a big factor as the pipe moves relatively slowly. However, this type of testing introduces another issue worth mentioning. What is the untested end of an EMAT probe/system. This can be easily verified using holes at regular intervals from the end of the pipe and observing how many of them are picked up by either an EMAT or UT system